Degeneration of the nigral-striatal dopaminergic pathway is a major characteristic of patients with Parkinson's disease. Presently the etiology of this progressive degenerative process is unknown. However, the symptoms of this disorder can apparently be remedied by increasing neurotransmitter levels in the surviving neurons. This is accomplished by administering high levels of the precursor L-Dopa. The present investigation is an attempt also to increase endogenous dopamine neurotransmission by increasing the metabolism of the rate-limiting enzyme tyrosine hydroxylase. Apparently this can be accomplished by raising the in vivo tissue levels of terahydrobiopterin (BH4). Our approach will be to administer large doses BH4 (2.5 mg/kg) of BH4 to patients while monitoring physiological responses as indicated in the protocol. Concurrent with this, CSF biopterin levels, DOPAC and HVA will be determined.